Glucose Tolerance Testing: Patient Preparation and Timed Collection

What Is the Oral Glucose Tolerance Test?

The oral glucose tolerance test (OGTT) measures how efficiently the body processes glucose over time. A physician orders the test to screen for or diagnose conditions such as type 2 diabetes, gestational diabetes, and impaired glucose tolerance. The two-hour OGTT is the most commonly performed version and requires multiple timed blood draws. Because the phlebotomist manages both specimen collection and timing, accuracy at every step directly affects patient results and diagnosis.

Pre-Test Patient Preparation

Proper preparation begins before the patient ever arrives. Patients must follow specific instructions for at least several days before the test:

• Dietary intake: Most protocols require patients to eat a normal carbohydrate-containing diet for a set number of days before the test. Patients should not restrict carbohydrates, as this can falsely elevate results. Always follow the ordering facility's specific protocol.
• Fasting period: Patients must fast for a defined period — typically eight to twelve hours — before arriving. Water is generally permitted; other beverages, food, gum, and tobacco are not.
• Medications: Patients should ask their physician whether to continue or hold medications that may affect glucose metabolism. The phlebotomist does not advise patients to stop or change medications — that is outside scope of practice.
• Physical activity: Patients are usually instructed to avoid strenuous exercise the day before and during the test.
• What to bring: Patients should bring their identification, insurance information, and the physician's order. Because the appointment lasts two hours or longer, advising patients to bring reading material or other quiet activity helps them remain calm and seated.

Step-by-Step OGTT Protocol

1. Confirm Fasting Status

When the patient arrives, ask directly whether they have fasted for the required time and have not eaten, drunk (other than water), chewed gum, or smoked. Document the patient's response. If the patient has not fasted correctly, do not proceed — notify the ordering physician or supervisor. Continuing with a non-fasted patient produces invalid results.

2. Collect the Fasting Specimen

Draw the baseline (fasting) blood specimen before any glucose is given. Label the tube with the correct time and mark it as the fasting draw. This specimen establishes the patient's baseline blood glucose.

3. Administer the Glucose Load

Immediately after the fasting draw, provide the standardized glucose beverage. The dose used is determined by the physician's order and the facility protocol — do not substitute or alter the beverage. Instruct the patient to drink the full amount within the time specified (commonly within five minutes). Record the exact time the patient finishes drinking; collection intervals are timed from this point.

4. Timed Collection Intervals

For a two-hour OGTT, specimens are collected at defined intervals after the glucose load — most commonly at one hour and at two hours. Some protocols also include a 30-minute draw. Each draw must occur within a narrow window of the scheduled time (typically within five minutes of the target time). Document the exact time of every draw on the specimen label and in the patient record.

5. Patient Restrictions During the Test

While waiting between draws, the patient must:

• Remain in the collection area
• Refrain from eating, drinking (except water as permitted by protocol), smoking, or exercising
• Avoid physical activity such as pacing or climbing stairs

Explain these requirements to the patient at the start of the appointment. Movement and eating can alter glucose metabolism and invalidate results.

6. Specimen Handling

Handle each specimen as directed by the laboratory. Many facilities use sodium fluoride (gray-top) tubes for glucose testing because the additive inhibits glycolysis and preserves glucose levels. Follow your laboratory's requirements. Process and transport specimens promptly according to protocol.

Safety and Scope-of-Practice Points

• Never proceed with a non-fasted patient without physician authorization. Document everything.
• Never alter the glucose dose or substitute a different beverage — this is outside the phlebotomist's scope.
• If a patient vomits the glucose beverage, do not re-administer it. Stop the test, document the incident with the exact time, and notify the ordering physician or laboratory supervisor immediately. The physician decides whether to reschedule.
• If a patient eats or drinks something other than water during the test, the test is compromised. Stop, document, and notify the supervisor or physician.
• Missed draw timing: If a draw is significantly late (the acceptable window varies by facility, but even a 10–15 minute delay can be problematic for some protocols), document the actual collection time precisely. Notify the laboratory and supervisor so the result can be interpreted in context. Never falsify a collection time.
• Monitor the patient for symptoms of hypoglycemia — sweating, dizziness, weakness, or fainting — during the waiting period. If symptoms occur, notify nursing staff or the ordering provider immediately.

Documentation Is Part of the Job

Accurate, complete documentation is as important as the draw itself. Record the fasting confirmation, the time the glucose load was finished, and the exact time of each specimen collection. Errors in timing documentation can cause correct results to be misinterpreted and harm the patient.